The present invention relates to a method of coating a substrate.
It has been proposed previously to provide various coatings which can be applied to a substrate in a liquid form and which can then be cured to form a solid coating. Typically, the liquid coating incorporates unsaturated organic compounds which include Cxe2x95x90C double bonds. These compounds present within the liquid coating are effectively polymerised during the curing process.
It has been proposed to effect the curing utilising high energy electron radiation. Typically electrons are delivered by an electron beam accelerator which normally operates at a voltage in excess of 150 kVe, although alternatively a nuclear source may be utilised. The radiation breaks some of the Cxe2x95x90C double bonds present in the unsaturated organic material, generating free radicals which initiate free radical polymerisation of the remaining material. The equipment necessary to carry out this process is costly to purchase and has to be specially shielded to avoid any leakage of gamma radiation.
There have been many proposals concerning coatings which can be cured, in response to ultra-violet light, involving free radical initiated polymerisation. Typically these coatings utilise a photo-initiator. A photo-initiator in this process is a material that absorbs light, and generates free radicals. The free radicals initiate the polymerisation of the coating. Photo-initiators are generally expensive, and can give rise to problems. For example, they can create an undesirable odour or taste (which may be relevant when the coating is going to be in contact with a food product) and can also cause xe2x80x9cyellowingxe2x80x9d, which is a tendency for the cured coating to adopt a yellow colour over the course of time.
It has been proposed to provide a coating which can be cured on exposure to ultra-violet light which does not incorporate a photo-initiator. A coating of this type is disclosed in U.S. Pat. No. 5,446,073. This Specification teaches a formulation which has a balance of xe2x80x9cacceptorxe2x80x9d and xe2x80x9cdonorxe2x80x9d species. The process described in U.S. Pat. No. 5,446,073 has not yet been adopted by industry, primarily because the reactive materials required are not readily commercially available. Also, the curing process is relatively slow with cure times typically being measured in terms of minutes. A further disadvantage of this technique is that it requires a combination of electron donating monomers and electron accepting monomers of relatively low molecular weight, and monomers in general are regarded as being prone to shrinkage during cure and are also regarded as being toxic since they may relatively easily penetrate the skin.
It has been discovered that short wavelength light may be used to effect a cure by direct fragmentation, in a similar way to the electron beam accelerator. Thus, it has been proposed to use light from excimer lamps, which have a wavelength of 172 nm, to cure radiation curable coatings without the use of a photo-initiator. However, this technique has only been used successfully with very thin coatings, typically coatings less than 1 xcexcm thick. The excimer energy is not able to penetrate readily into a coating which is of a greater thickness without excessive heat being generated. If an excimer lamp were used to irradiate a 10 xcexcm thick coating, it would produce a xe2x80x9ccured skinxe2x80x9d on the surface, but not a complete cure.
The present invention seeks to provide a UV cured coating in which the disadvantages of prior proposals are obviated or reduced.
According to one aspect of the present invention, there is provided a method of coating a substrate, the method comprising the steps of applying a coating composition to at least selected areas of the substrate, exposing the coated substrate to ultra-violet light from at least one lamp having a power output of at least 140 watts per linear centimeter in a curing zone, to initiate curing of the coating, the coating composition comprising a mixture including at least a reactive part comprising between 30% and 100% multi-functional material and being photo-initiator-free, including the step of maintaining a substantially inert atmosphere in the curing zone where the substrate is exposed to said ultra-violet light.
The preferred multi-functional materials have a functionality of at least three.
Preferably, the inert atmosphere is obtained by purging the curing zone with inert gas such as nitrogen.
Advantageously, the oxygen concentration in the curing zone is less than 1,000 ppm and preferably less than 100 ppm.
Preferably, the multi-functional material comprises one or more reactive diluents.
Conveniently, the multi-functional material comprises one or more materials, each material having a molecular weight in excess of 480.
Advantageously, the multi-functional material comprises one or more materials which have three or more functional acrylate groups.
Conveniently, the coating material contains a pre-polymer, and may comprise polyester acrylate, polyurethane acrylate, epoxyacrylate or a full acrylic material.
Conveniently, the pre-polymer is multi-functional.
Advantageously, the coating composition comprises, in addition to the reactive part, a filler, and the filler may comprises clay, silica or magnetisable particles.
Preferably, the power output of the lamp is at least 180 watts/cm and may be substantially 240 watts/cm.
Conveniently, UV light from the lamp has a substantial spectral content in the range 200-300 nm.
Preferably, UV light from the lamp has additional spectral content with peaks of approximately 370 nm, 408 nm and 438 nm.
Two or more lamps may be provided in the curing zone. The lamps may have different spectral properties or may have substantially identical spectral properties.
The invention relates to a substrate when coated by a method as described above.